It is well known that carbon nanotubes (CNTs) possess ultrahigh thermal
conductivity that is comparable to bulk diamond. However, no research
has studied the possible low thermal conductivity of different CNTs so
far. By performing nonequilibrium molecular dynamic simulations, we
reveal that the perfect graphyne nanotube (GNT) exhibits an
unprecedentedly low thermal conductivity (below 10 W/mK at room
temperature), which is generally two orders of magnitude lower than that
of ordinary CNTs and even lower than the values reported for defected,
doped, and chemically functionalized CNTs. By performing phonon
polarization and spectral energy density analysis, we observe that the
ultralow thermal conductivity stems from the unique atomic structure of
the GNT, consisting of the weak acetylenic linkage (sp C-C bonds) and
the strong hexagonal ring (s p(2) C-C bonds), which results in a large
vibrational mismatch between these two components, and thus induces
significantly inefficient heat transfer. Moreover, the thermal transport
in GNT with a large number of acetylenic linkages is dominated by the
low frequency longitudinal modes in the linkage. Such strong confinement
of the low frequency thermal energy results in the extremely low thermal
conductivity due to the flattened phonon dispersion curves (low phonon
group velocities). The exploration of the abnormal thermal transport of
GNTs paves the way for design and application of the relevant devices
that could benefit from the ultralow thermal conductivity, such as
thermoelectrics for energy conversion.